Ink jet recording has been popularized rapidly and will see further development because of low material cost, high speed, low-noise, and ease of full color recording. Fundamentally, ink jet recording is divided into a continuous method in which ink droplets are continuously allowed to fly and a drop-on-demand method in which ink droplets are ejected in response to image information signals. The mechanism of drop formation and ejection includes a piezoelectric system in which pressure is applied to ink by a piezoelectric element to eject ink droplets, a thermal system in which heat is applied to ink to create bubbles whereby to eject an ink droplet, an ultrasonic system, and an electrostatic system. Ink jet inks include aqueous ink, oily ink, and hot-melt or solid ink.
Colorants used in the ink-jet inks are required to have (1) good solubility in ink solvents, e.g., water, (2) capability of high-density recording, (3) satisfactory hues, (4) color fastness against light, heat, active gases in the atmosphere (e.g., NOx, oxidizing gases such as ozone, and SOx), (5) resistance against water or chemicals, (6) good fixability on image receiving media with minimized feathering, (7) stability in ink formulations, (8) non-toxicity, (9) high purity, and (10) inexpensiveness.
In particular, colorants for ink jet recording are strongly required to be fast to light, humidity and heat, and, when used in printing a substrate having an ink receiving layer containing white, porous, inorganic pigment particles, to exhibit resistance against environmental oxidizing gases such as ozone, and to have high water resistance.
Among defects of ink jet prints is bronzing, a visual effect observed in areas of high optical density in which the surface of a recorded image reflects light to take on a metallic appearance because the colorant crystallizes on the print surface as it dries. Bronzing tends to occur when a colorant is made less water-soluble or when a hydrogen bond is introduced into a colorant structure so as to improve print resistance to water, light or gas. The light reflection and scattering due to bronzing not only reduces the optical density of the image but also varies the hue of the recorded image from what is expected and impairs the transparency of the image. Freedom from bronzing is one of important performance properties required of ink jet ink.
Known means for suppressing bronzing include (a) addition of a specific nitrogen compound (see JP-A-6-25575, JP-A-6-228476, JP-A-6-248212, JP-A-7-228810, JP-A-7-268261, JP-A-9-12946, and JP-A-9-12949), (b) addition of a specific heterocyclic compound (see JP-A-8-259865), (c) addition of a specific titanium compound (see JP-A-8-337745), and (d) addition of an alkali metal ion (see JP-A-7-26178). These additives succeed in controlling development of bronzing but raise a different problem. Some of them should be used in a large quantity because of insufficient effect, and some others can deteriorate ink properties and image qualities, such as storage stability of ink. According to JP-A-8-259865, for example, addition of an alkanolamine to ink prevents bronzing, but addition of only a small amount of an alkanolamine results in an increase of ink pH to 11 or higher. Such a high pH ink not only affects ink jet nozzles but lacks safety in case of contact with skin and also reduces print quality and water resistance of prints.
Likewise the additives heretofore proposed, while effective in various aspects, meet difficulty in producing the expected results while retaining the performance properties of ink formulations. Where the solubility and association of a colorant should be taken into due consideration, selection of the kind and amount of an additive would be understandably difficult. When an ionic additive is used, the influences of the counter ion must be considered. It has therefore been demanded to explore the molecular design of an additive based on an innovative idea and to introduce a method for essentially controlling bronzing with the additive.